I don't think this has anything to do with harness line placement or where the mast foot is placed. I have an iS 111 and TR Maui race Sails and have experienced the same problem. When rigged to specifications, the tack strap attaches to the middle of the universal tendon with the foot of the sail almost touching the board.
Simply rig with a few more centimeters on your extension to raise the sail a little higher off the deck of the board.
Harness line should be attached where the center of force is when fully powered. Move them back and the front hand has too much pull. I guess if you have an adjustable outhaul, you can move the lines back and flatten the sail, but then you may cause your rig to depower and become twitchy.
Your 38cm fin may be a little small too, but that depends if you are free sailing or downwind slalom. I weigh 78 kg and use a Tectonics Tomcat 42, and it works great for freesailing. Yesterday, I was on the iS 111 with a TR 4 6.6 with the 42 fin in 18-22 knots and had no problem with the fin being too large. Just free sailing and hit a peak speed of 30.5 knots on one full reach in a slightly protected area (smaller chop).
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.